The objectives of this study are to explore the role of adenosine in autoregulation of the normal coronary circulation in man, and as an indicator in myocardial ischemia. Work in animals has firmly established that adenosine is involved in the autoregulation of coronary blood flow, and has shown that concentrations of this compound and its breakdown products increase markedly in the coronary sinus blood during induced ischemia. We have previously demonstrated that the product of heart rate and blood pressure is a good index of myocardial oxygen requirements during exercise in man. Data will be collected from patients who are undergoing exercise during diagnostic cardiac catheterization. Simulataneous measurements of heart rate, blood pressure and the concentrations of adenosine and its breakdown products in coronary sinus blood will be madr at several work loads. In order to establish the normal relationship between these variables measurements will be made in patients with lesions that do not involve an abnormality of left ventricular loading or blood supply. The same measurements will be made in patients during angina pectoris and these data will be compared to the normal values in order to determine if an increase in the concentration of adenosine (and/or its metabolites might serve as a more sensitive indicator of myocardial ischemia than measurements currently used for research or clinical purposes.